1. Field of the Invention
The present invention relates to a carrier for electrostatic development (a carrier for electrophotography) used for electrostatic latent image in electrophotography and electrostatic recording and charged member such as a sleeve and coating member for electrostatic development and the like, as well as an electrostatic latent image developer utilizing the coating member.
2. Description of the Related Art
A method for visualizing an image information via electrostatic image such as electrophotography has been used in various fields. In electrophotography, an electrostatic latent image is formed on a photoreceptor in electrostatic charging and exposure steps, the electrostatic latent image is developed with a developer containing a toner, and is visualized via transfer and fixing steps. Examples of the developer used herein are a two-component developer composed of a toner and a carrier, and a one-component developer such as a magnetic carrier which is used alone. The two-component developer is now widely employed due to good controlling properties since a carrier performs functions such as stirring, transportation and electrostatically charging of a developer, which are separated into each component in the developer.
In addition, as a developing method, a cascade method or the like was conventionally used, while a magnetic brush method is predominant at present, where a magnetic roll is utilized as a developer conveying carrier. A conductive magnetic brush (CMB) development utilizing a conductive carrier and an insulating magnetic brush (IMB) development utilizing an insulating carrier are known as a two-component magnetic brush development. Among them, the CMB development has characteristics such that charge is injected through a developing roll due to lower resistance of a carrier, carriers in the vicinity of a photoreceptor plays a role as a developing electrode to increase the effective developing electric field and, as a result, transfer of a toner is sufficiently performed, resulting in the excellent productivity of a solid image. On the contrary, there is a problem that image defects are easily produced, such as a white line called brush mark caused by charge injection through a developing roll and the like and transfer of a carrier to a photoreceptor called carrier-over and the like.
Recently, coloring technique has rapidly progressed, and higher level has been required on color image quality. In particular, solid image is important in the color image quality. Therefore, the CMB carrier having improved performance including durability is strongly desired. Japanese Patent Application Publication (JP-B) No. 7-120086 discloses a carrier which abruptly changes in its resistance at a certain electric field by coating a core material (hereinafter referred to as "carrier core" or "core") having relatively low resistance with a resin having higher resistance, and thus becomes higher resistance at a lower electric field and lower resistance at higher electric field. It is described for the carrier that excellent black solid print is obtained without carrier-over at a non-latent image part since higher electric field is applied to a latent image part and lower electric field is applied to a non-latent image part. However, it is presumed that, from the description in Examples and Actions in the specification of the invention (JP-B No. 7-120086), the thickness of a resin coating layer is significantly small, and a low resistive core is partially exposed. It is considered that such a structure makes the carrier lower resistance at higher electric field. In fact, as described in Comparative Examples below, in case where the core is completely coated with the resin and the thickness of a resin coating layer is large, the carrier is higher resistance even at higher electric field to provide no good solid image. In a partial coating where a part of such a low resistive core is exposed, charge is easy to move through an exposed surface, whereby brush mark is easy to be produced at a latent image part.
In addition, Japanese Patent Application Laid-Open (JP. A) Nos. 61-107257 and 61-13059 disclosed ferrite having relatively low resistance and having irregularity on the surface based on primary particles. It is described that leakage between different polar charges is suppressed to prevent brush mark due to such fine irregularity. However, there is a problem that, because of fine irregularity on the carrier surface, an area in contact with a toner is increased, and thus a toner tends to adhere to the carrier with decreasing an ability of the carrier to be charged over a period of time. In addition, JP-A No. 61-161157 disclosed an invention defined by a ratio of resistance of a carrier core and that of a carrier obtained by coating a resin thereon. It is shown therein that such definition satisfies all the requirement such as degree, solid image concentration and fine line productivity at a time. However, the invention provides insufficient effect in terms of preventing image defects for a color image.
On the other hand, there are a variety of properties required for a carrier coated with a resin, and it is necessary to impart suitable electrostatically charging property (the amount and distribution of electrostatic charge) to a toner and the suitable electrostaticall charging property of a toner is required to be maintained over a longer period of time. Properties such as resistance to impact and resistance to abrasion are required for the carrier. In particular, it is important that electrostaticall charging property of a toner is not sensitive to changes in enviromental condition such as humidity, temperature and the like. Thus a variety of carriers coated with a resin have been proposed.
More particularly, there are proposed the use of a copolymer of nitrogen containing alkyl (meth)acrylate and vinyl monomer, and a copolymer of fluorinated alkyl (meth)acrylate and vinyl monomer (see JP-A Nos. 64-35526 and 2-24670).
In addition, it is disclosed that the surface of a carrier core was coated with a copolymer of a nitrogen-containing monomer and a fluorinated monomer to obtain a carrier coated with a resin having relatively long life (see JP-B No. 3-23909). However, it is difficult to obtain uniform composition because of difficulty in copolymerizing monomers or phase separation. Furthermore, since the composition has a certain wide distribution, deviation of properties is caused between coated portions and un-coated portions to unsufficiently provide resistance to impact and resistance to staining for a toner. In particular, there was a problem that, since the reduced amount of electrostatic charge at high temperature and high humidity and the extremely increased amount of electrostatic charge at low temperature and low humidity make stability of electrostatically charging property of a developer lower, the carrier did not withstand over long-time use because of fog in an image and unevenness in image concentration being occurred.
In addition, a carrier having low surface energy has been disclosed, which is coated with a coating layer containing a silicone resin. Examples may include a carrier having the surface coated with a mixture of an unsaturated silicone resin and an organosilicone, silanol and the like with styrene/acrylic resin (U.S. Pat. No. 3,562,533), a carrier having the surface coated with polyphenylene resin and organosilicone terpolymer resin (U.S. Pat. No. 3,487,127), a carrier having the surface coated with styrene/acrylate or methacrylate resin and organosilane, silanol, siloxane and the like (U.S. Pat. No. 3,627,522), a carrier coated with a coating layer containing a silicone resin and a nitrogen-containing resin having electrostatically charging property (JP-A No. 55-127567), a carrier coated with a resin-modified silicone resin (JP-A No. 55-157751), and a carrier coated with a mixture of a copolymer of acrylic ester and vinyl silane and a copolymer of acrylic ester containing fluorine-containing alkyl group and acrylic ester at a specified ratio (JP-A No. 2-34670) and the like.
A toner does not tend to adhere to a carrier coated with a silicone resin toner or a coating layer containing a silicone resin due to low surface energy thereof, and adhesive force to a charged member is not strong. Therefore, a coating layer is peeled off from an adhesion interface because of friction force, impact force, shear force and the like generated in a developing machine. Electrostatically charging property and electrical resistance are changed, and thus image quality is deteriorated. This tendency is remarkable at a higher copying rate of a copier and an increased stress applied to a charged member. In addition, also in a case where an organic pigment, a dye or the like is utilized as a colorant in a color toner, the toner tends to adhere to a carrier. Further, electrostaticall charging property and electrical resistance are changed, and thus image quality is deteriorated at a higher copying rate of a copier and an increased stress applied to a charged member. The phrase "the toner adhering to the carrier" or the term "spentation" used herein refers to phenomenon in which firm adherence or fusion of a toner and/or an additive, colorant and the like to the surface of charged member occurs because of mechanical collision such as collision and friction between toner particles, or collision and friction between toner particles and a developing machine, as well as exothermic heat caused by friction.
In addition, in a carrier coated with a mixture of acrylic ester containing fluorine-containing group and a copolymer with acrylic ester at a specified ratio, since resins having different surface tension from each other are mixed in a solution, a solution is evaporated during coating to a carrier. As an amount of resin solid grows larger on the surface, a coating is formed such that the coating has a domain structure (sea-island structure) and in which phase separation is caused, leading to a problem on resistance to impact and adhesive property.
Therefore, there has been a need for a carrier and the like which suppresses "spentation", enhances adhesive interface strength of a coating layer and resin strength of a coating layer, and has stability of electrostatically charging for long-time use.